The present invention relates to an improved apparatus for detecting a relative rotational position between two shafts, which is suitable for use as, for example, a torque sensor for detecting a torsional force or load applied to a power steering shaft of a motor vehicle.
Of various types of techniques for detecting torsional amounts of two relatively rotatable shafts, there have been well known those which are characterized by provision of detection devices, such as a potentiometer or resolver devices, on input and output shafts interconnected via a torsion bar. According to the above-mentioned technique using a potentiometer, a slider is mounted on the input shaft while a resistor is mounted on the output shaft, so that a position of the slider contacting the resistor varies in accordance with a variation in a relative rotational position between the input and output shafts to thereby provide an analog voltage corresponding to the relative rotational position. According to the technique using resolver devices, separate resolver devices are provided on both of the input and output shafts so as to detect a relative rotational amount (torsional amount) between the two shafts on the basis of angle signals produced by the two resolver devices. Further, as a means for detecting a relative rotational displacement between two relatively rotatable shafts, there has been developed a noncontact-type torque sensor for electronic power steering which employs an induction coil.
The conventional technique of the type using a potentiometer would always suffer from poor electrical contact, failure and/or other problem since the electrical contact is implemented via a mechanical contact structure. Further, because there occurs impedance variations due to temperature changes, it is necessary to appropriately compensate for a temperature drift. Further, the rotational-displacement detection apparatus, known as the noncontact-type torque sensor for electric power steering employing the induction coil, is arranged to measure an analog voltage level produced in response to a minute relative rotational displacement, so that it only accomplishes a very poor detecting resolution. Further, in addition to the need to compensate temperature drift characteristics of the coil, there is a need to appropriately compensate temperature drift characteristics present in reluctance of magnetic substances that vary magnetic coupling to the coil in response to a changing relative rotational position as well as in eddy current loss of electrically conductive substances. Furthermore, it is desirable that the torque sensors for motor vehicles be arranged as a dual-sensing structure for safety purposes.
It is therefore an object of the present invention to provide a relative-rotational-position detection apparatus which can achieve superior temperature-characteristic compensating performance and can be easily arranged as a dual-sensing structure.
It is another object of the present invention to provide a relative-rotational-position detection apparatus which is capable of high-resolution detection even when a relative rotational displacement to be detected is very minute.
According to an aspect of the present invention, there is provided a relative-rotational-position detection apparatus for detecting a relative rotational position between a first shaft and a second shaft rotatable relative to each other, which comprises: an outer cylindrical section rotatable with the first shaft, the outer cylindrical section being formed of a magnetic-shielding substance or antimagnetic substance and having patterns of open windows of first to fourth channels; an inner cylindrical section inserted in the outer cylindrical section and rotatable with the second shaft, the inner cylindrical section being formed of a magnetic substance and having patterns of nonmagnetic windows of the first to fourth channels; and first to fourth coils provided in corresponding relation to the first to fourth channels and excitable by a predetermined A.C. signal. For each of the channels, the pattern of the open window in the outer cylindrical section and the pattern of the nonmagnetic window in the inner cylindrical section are arranged to overlap with each other, and an arrangement (i.e., layout) of the open windows and nonmagnetic windows in the channels is set such that: degree of overlap between the pattern of the open window and the pattern of the nonmagnetic window in each of the channels varies in accordance with a variation in a relative rotational position, within a predetermined range, between the first shaft and the second shaft; a variation in the degree of overlap in the second channel presents a differential characteristic relative to a variation in the degree of overlap in the first channel; a variation in the degree of overlap in the fourth channel presents a differential characteristic relative to a variation in the degree of overlap in the third channel; and variations in the third and fourth channels present a predetermined difference from variations in the first and second channels. Thus, in the present invention, the first to fourth coils corresponding to the first to fourth channels can present impedance corresponding to the degree of overlap between the open windows and the nonmagnetic windows of the corresponding channels.
The relative-rotational-position detection apparatus of the present invention, which is characterized by the provision of four detecting channels each comprising a combination of the relatively-displaceable open window and nonmagnetic window, is extremely useful in that it permits accurate detection by appropriately compensating temperature drift characteristics and in that it can be constructed to provide dual detection outputs.
In a preferred implementation, a first A.C. output signal is generated by obtaining a difference between outputs of the first coil corresponding to the first channel and the second coil corresponding to the second channel, a second A.C. output signal is generated by obtaining a difference between outputs of the third coil corresponding to the third channel and the fourth coil corresponding to the fourth channel, and the arrangement of the open windows and nonmagnetic windows of the individual channels is set in such a manner that the amplitudes of the first and second A.C. output signals vary with different characteristics in accordance with a variation in the relative rotational position within the predetermined range. By thus obtaining a difference between two channels, it is possible to cancel out temperature drift errors in the coil impedance. Further, the first and second A.C. output signals can be used as dual detection outputs.
The relative-rotational-position detection apparatus may further comprise a first circuit for synthesizing the first and second A.C. output signals to thereby generate an A.C. signal representative of a phase corresponding to the relative rotational position, and a second circuit for selecting one of the first and second A.C. output signals. Here, the relative rotational position can be detected either on the basis of the A.C. signal representative of a phase corresponding to the relative rotational position obtained by the first circuit or on the basis of an amplitude level of the one of the first and second A.C. output signals selected by the second circuit. For example, in normal cases, the relative rotational position may be detected by a phase detection scheme with higher accuracy, using the first A.C. output signal. In case there has occurred an anomaly in one of the first and second A.C. output signals, the second circuit may select the other of the first and second A.C. output signals which has no anomaly, so that the relative rotational position can be detected on the basis of an amplitude level of the A.C. output signal selected by the second circuit. In this way, the relative-rotational-position detection apparatus of the present invention can reliably satisfy requirements pertaining to both detection accuracy and safety.
According to another aspect of the present invention, there is provided a relative-rotational-position detection apparatus for detecting a relative rotational position between a first shaft and a second shaft rotatable relative to each other, which comprises: an outer cylindrical section rotatable with the first shaft, the outer cylindrical section being formed of a magnetic-shielding substance or antimagnetic substance and having patterns of open windows of first and second channels; an inner cylindrical section inserted in the outer cylindrical section and rotatable with the second shaft, the inner cylindrical section being formed of a magnetic substance and having patterns of nonmagnetic windows of the first and second channels; and a coil section including at least two coils for each of the first and second channels and excitable by a predetermined A.C. signal. For each of the channels, the pattern of the open window of the outer cylindrical section and the pattern of the nonmagnetic window of the inner cylindrical section are arranged to overlap with each other, and an arrangement of the open windows and nonmagnetic windows in the channels is set such that degree of overlap between the pattern of the open window and the pattern of the nonmagnetic window in each of the channels varies in accordance with a variation in a relative rotational position, within a predetermined range, between the first shaft and the second shaft and a variation in the degree of overlap in the second channel presents a predetermined difference from a variation in the degree of overlap in the first channel. Here, the coils corresponding to the first and second channels present impedance corresponding to the degree of overlap between the open windows and the nonmagnetic windows in the corresponding channels.
While the described embodiments represent the preferred form of the present invention, it is to be understood that various modifications will occur to those skilled in the art without departing from the spirit of the invention. The scope of the present invention is therefore to be determined solely by the appended claims.